An Innovative Acid Diversion Using In-Situ Foam Generation: Experimental and Successful Field Application

In carbonate reservoirs, effective acid stimulation is essential to overcome reservoir damage and mainline high oil production. Recently, most of oil wells are being drilled horizontally to maximize production. Acid stimulation of horizontal wells with long intervals require very effective acid diversion system. If the diversion system is not efficient enough, most of the acid will be leaking-off near the casing shoe, in openhole well, which will result in a fast water breakthrough and diminish production. This study describes a breakthrough treatment for acidizing long horizontal wells in carbonate formations. The novel technology is based on in-situ foam generation to divert the acid. Gas diversion, as a foam, is a perfect diversion mechanism as gas creates pressure resistance which forces the acid stages to be diverted to new ones?. The diversion will not require the acid to be spent, compared to viscoelastic diverting system. Moreover, no gel is left behind post treatment, which will eliminate any damage potential. The system is not impacted with the presence of corrosion products, where diverting system will not function without effective pickling and tubular cleanup. Lab results showed that the new in-situ foam generation system was very effective on both dolomite and calcite cores. The system creates high back pressure when foam is generated, which significantly diverts the acid stages to stimulate other intervals. Moreover, the new system minimizes acid leak-off and penetration. Open completing the job, the foam collapse leaving no left behind any damaging material. Field application of the in-situ foam generating system showed high success rate and outperformed other diversion mechanisms. The well gain was up to 18 folds of the original well injectivity.

Successful Implementation of a Modified Carbonate Emulsion Acid System Combined with an Engineered Diversion Approach Delivers Outstanding Results – A Case Study from the Moho Nord Deep Offshore Field in Congo

Moho Nord deep offshore field is located 80 kilometers offshore Pointe-Noire in the Republic of the Congo. The wells produce crude from the Albian age reservoir and lithology consists of alternating sequences of carbonates and sandstone layers with high heterogeneity and permeability contrast, including the presence vacuolar layers called "hyperdrains". This paper describes the application of a novel acid system and the methodology successfully applied to effectively acid stimulate the Albian drain. The combination of long perforation intervals with lithology and permeability contrasts, natural fractures, and the potential for asphaltene deposition resulted in adoption of a Modified Carbonate Emulsion Acid (MCEA) fluid system containing a solvent to provide asphaltene deposition prevention. The MCEA stimulation treatments were bullheaded from a stimulation vessel and an engineered diversion process was implemented for effective acid diversion using a combination of mechanical ball sealers and a degradable particle system (DPS). The selection of number of ball sealers and the DPS diverter design depended upon the interpretation of zone permeability profile from the logs, and the final distribution of perforations selected along the drain. A fluid placement simulator indicated low sealing efficiency of the ball sealers would lead to an overstimulation of the highest permeability areas. Subsequent simulations indicated that the DPS would provide better acid coverage with lower skin (S). Results and observations presented indicate that the decision to improve the acid diversion design and combine ball sealers with a DPS diversion technique to improve zonal coverage was validated. During the stimulation treatment execution, the high stimulation treatment efficiency was clearly apparent from the pressure responses to the acid and the diverter system which sealed off perforations and diverted the treatment to other layers with lower permeability. The MCEA also has proven to have self-diverting properties due to its high viscosity and low reaction rate which creates a better coverage of the drain, even with limited pumping rate, allowing live acid penetrating deeper into the formation. The production results reported from the 15 wells stimulation campaign (10 producers, 5 injectors) indicated that the productivity indexes (PI) exceeded expectations and resultant post-stimulation skin values ranged from −2.5 to −4.1. The Moho Nord deep offshore stimulation campaign yielded outstanding production results and showed significant validation for use of the MCEA system and the diversion methodology applied. On the producer wells the use of both chemical and mechanical diversion was valuable, as the DPS proved to complement the Ball Sealers for layers with lower injectivity and also at the high injection rates. High injectivity gain coupled with effective diversion was crucial for enhanced wormholing and good drain coverage.

Effect of fiber on the rheological properties of gelled acid

Degradable fiber is widely used to assist gelled acid diversion and reduce acid leak-off in the acid stimulation of carbonate hydrocarbon reservoirs in Sichuan Basin of China. The rheological properties of an acid system will affect the geometry of the acid-etching fracture. However, the effect of fiber on the rheological properties of gelled acid is not yet clear. This paper investigates the rheological properties of gelled acid with various fiber concentrations at different temperatures. The results show that when the temperature is less than the degradable temperature of the fiber, the apparent viscosity of gelled acid rises gradually with an increase in fiber concentration, while the fiber has no significant effect on the viscosity of gelled acid at the degradable temperature. The dissolution process of fiber in gelled acid experiences none-dissolution, surface dissolution, dissolution and fining, and a complete dissolution stage from low to high temperatures, which all have different effects on gelled acid viscosity. The fiber links more gelling agent molecules of gelled acid together to form a quasi-network structure between the fiber and fiber and the fiber and polymer, which results in a rise in the viscosity of gelled acid. The acid system also shows a strong shear thinning property under different temperatures and fiber concentrations. However, the power-law index n of this acid system always maintains a steady average value of about 0.181, while the change pattern of consistency index K is similar to the change in viscosity with varying fiber concentrations and temperatures. The research results are useful for acid fracturing treatment design in carbonate reservoir.

A Novel Approach to Improve Acid Diversion in Carbonate Rocks Using Thermochemical Fluids: Experimental and Numerical Study

The distribution of acid over all layers of interest is a critical measure of matrix acidizing efficiency. Chemical and mechanical techniques have been widely adapted for enhancing acid diversion. However, it was demonstrated that these often impact the formation with damage after the acid job is completed. This study introduces, for the first time, a novel solution to improve acid diversion using thermochemical fluids. This method involves generating nitrogen gas at the downhole condition, where the generated gas will contribute in diverting the injected acids into low-permeability formations. In this work, both lab-scale numerical and field-scale analytical models were developed to evaluate the performance of the proposed technique. In addition, experimental measurements were carried out in order to demonstrate the application of thermochemical in improving the acid diversion. The results showed that a thermochemical approach has an effective performance in diverting the injected acids into low-permeability rocks. After treatment, continuous wormholes were generated in the high-permeability rocks as well as in low-permeability rocks. The lab-scale model was able to replicate the wormholing impact observed in the lab. In addition, alternating injection of thermochemical and acid fluids reduced the acid volume 3.6 times compared to the single stage of thermochemical injection. Finally, sensitivity analysis indicates that the formation porosity and permeability have major impacts on the acidizing treatment, while the formations pressures have minor effect on the diversion performance.

A New Cationic Polymer System That Improves Acid Diversion in Heterogeneous Carbonate Reservoirs

Summary In-situ gelled acids are used for acid diversion in heterogeneous carbonate reservoirs. However, most of the gelled systems are based on anionic polymers that are difficult to clean up after the acid treatments. Residual polymer deposition leads to formation damage by blocking pore throats in the matrix. This work evaluates a new cationic-polymer acid system with self-breaking ability for application as an acid diverter in carbonate reservoirs. Experimental studies have been conducted to examine the rheological properties of these polymer-based acid systems. The apparent viscosities of the live and the partially neutralized acids at pH from 0 to 5 were measured against the shear rate (0 to 1000 s−1). The effects of salinity and temperature (80 to 250°F) on the rheological properties of the acid system were also studied. The viscoelastic properties of the gelled acid system were evaluated using an oscillatory rheometer. Dynamic sweep tests were used to determine the elastic (G′) and viscous (G″) moduli of the system. Single-coreflood experiments were conducted on Indiana limestone cores to study the nature of diversion caused by the polymer-acid system. The effect of permeability contrast on the process of diversion was investigated by conducting dual-coreflood experiments on Indiana limestone cores that had permeability contrasts of 1.5 to 20. Computed tomography (CT) scans were conducted to study wormhole propagation after acid injection for both single and dual cores. The live acid system displayed a non-Newtonian shear-thinning behavior with the viscosity declining as temperature increased. For 5 wt% hydrochloric acid (HCl) and 20 gal/t polymer content at 10 s−1, the viscosity decreased from 230 to 40 cp as the temperature increased from 88 to 250°F. Acid-spending tests demonstrated that the acid generated a gel with improved viscosity of 260 cp (at 250°F and 10 s−1) after it reached a pH of 2. The highly viscous gel plugged the wormhole and forced the acid that followed to the next higher-permeability zone. The viscosity of the gel continued to increase until it broke down to 69 cp (at 250°F and 10 s−1) at a pH of 4.8, which indicates a self-breaking system and more thorough cleanup potential. Coreflood studies indicated that the wormhole and the diversion process are dependent on the temperature and the flow rate. There was no indication of any damage caused by the system. The injected acid pore volume to breakthrough (PVBT) decreased from 2.2 to 1.4 when the temperature increased from 150 to 250°F. The strong elastic nature of the gel (G′ = 3.976 Pa at 1 Hz) formed by the partially neutralized acid system proves its suitability as a candidate for use as a diverting agent. This new acid-polymer system has significant promise for use in acid diversion to improve stimulation of carbonate reservoirs.